Postage On Large Christmas Cards

[hellywelly4]

I made the mistake of buying large Christmas cards because they were on offer. Now I've realised that I will have to pay extra to post them. I can't get to the Post Office but we have a P.O. box that I can pop them in. They are 7in by 10in. Would it be OK for me to put a first class stamp and a second class one on each envelope?
Advice appreciated.

[hellywelly4]

follyfancier - I shop with Tesco on line and they only have the normal size stamps available.